The processing of silicon wafers for semiconductor applications typically includes photolithography as one of the process steps. In photolithography, a wafer surface with a deposit of silicon nitride is coated over with a light-sensitive liquid polymer or photoresist and then selectively exposed to a source of radiation using a template with a desired pattern. Typically, ultraviolet light is shone through or reflected off a surface of a mask or reticle to project the desired pattern onto the photoresist covered wafer. The portion of the photoresist exposed to the light is chemically modified and remains unaffected when the wafer is subsequently subjected to a chemical media that removes the unexposed photoresist leaving the modified photoresist on the wafer in the exact shape of the pattern on the mask. The wafer is subjected to an etch process that removes the exposed portion of the nitride layer leaving a nitride pattern on the wafer in the exact design of the mask.
The industry trend is towards the production of chips that are smaller and/or with a higher logic density necessitating even smaller line widths on larger wafers. Clearly, the degree of fineness to which the surface of the reticle can be patterned and the degree to which this pattern can be faithfully replicated onto the wafer surface are factors that impact the quality of the ultimate semiconductor product. The resolution with which the pattern can be reproduced on the wafer surface depends on the wavelength of ultraviolet light used to project the pattern onto the surface of the photoresist-coated wafer. State-of-the-art photolithography tools use deep ultraviolet light with wavelengths of 193 nm, which allow minimum feature sizes on the order of 100 nm. Tools currently being developed use 157 nm Extreme Ultraviolet (EUV) light to permit resolution of features at sizes below 70 nm. The reticle is a very flat glass plate that contains the patterns to be reproduced on the wafer. Typical reticle substrate material is quartz. Because of the tiny size of the critical elements of modern integrated circuits, it is essential that the operative surface of the reticle (i.e. the patterned surface) be kept free of contaminants that could either damage the surface or distort the image projected onto the photoresist layer during processing leading to a final product of unacceptable quality. Typically, the critical particle sizes are 0.1 μm and 0.03 μm for the non-patterned and patterned surfaces respectively when EUV is part of the photolithography process. Generally, the patterned surface of the reticle is coated with a thin, optically transparent film, preferably of nitrocellulose, attached to and supported by a frame, and attached to the reticle. Its purpose is to seal out contaminants and reduce printed defects potentially caused by such contamination in the image plane. However, extreme EUV utilizes reflection from the patterned surface as opposed to transmission through the reticle characteristic of deep ultraviolet light photolithography. At his time, the art does not provide pellicle materials that are transparent to EUV. Consequently, the reflective photomask (reticle) employed in EUV photolithography is susceptible to contamination and damage to a far greater degree than reticles used in conventional photolithography. This situation imposes heightened functional requirements on any reticle SMIF pod designed to store, transport and ship a reticle destined for EUV photolithography use.
It is well known in the art that unnecessary and unintended contact of the reticle with other surfaces during manufacturing, processing, shipping, handling, transport or storage will likely cause damage to the delicate features on the patterned surface of the reticle due to sliding friction and abrasion. Likewise, it is generally accepted by those skilled in the art that any particulate contamination of the surface of the reticle can potentially compromise the reticle to a degree sufficient -to seriously affect the end products of processes that use such a flawed reticle. In this regard, the art has developed innovative approaches to locate and support the reticle in reticle containers so as to reduce or eliminate sliding friction and consequent abrasion of the reticle and the resultant generation of contaminating particulates. In recognition of the need to maintain a controlled environment around the wafer during storage, processing and transport, the prior art has evolved approaches to isolation technology that allows for control of the environment in the immediate vicinity of a wafer by providing for a container so that it can be kept relatively free from incursion of particulate matter. Typically, containers are provided with standardized mechanical interfaces that allow automatic manipulation of the container by processing machinery. Such containers can hold photomasks of up to 200 mm and are designated standard mechanical interface pods, or SMIF-Pods. Even with such a controlled environment, migration of particulates that may be present inside the controlled environment is still possible due to pressure changes of the air trapped in the controlled environment or turbulence of the trapped air brought on by rapid movements of the container and/or by disturbing the trapped air volume. For example, thin walled SMIF pods may experience wall movement due to altitude related pressure changes causing the trapped air inside the controlled environment to be displaced. Temperature changes can set up convection currents within the container. Dimensional changes of the container and its components due to pressure fluctuations can lead to compromising the sealing between cover and door of the carrier and incursion of particulates within the carrier. Prior art approaches contemplate a breathing apparatus between the external environment and the internal controlled volume of air. The breathing apparatus provides a path for the air to flow. Prior art breathing apparatus may include a particulate filter to block the entry of particulates from the external environment into the controlled environment of the carrier.
Those skilled in the art will appreciate that particulate contaminants are but one half of the equation. Equally important are gas-phase contaminants or airborne molecular contaminants (AMC) due to ambient air venting or leaking into or getting trapped in a hermetically sealed system. For example, at a suitable dew point temperature, the moisture in the air will condense out of the air and some of it may get deposited onto the reticle. Even with a perfectly sealed container, there is the possibility of air entering into the system when the reticle is removed from and replaced within the container during processing. Water vapor condensing onto the patterned surface of the reticle can interfere with the optics just as a solid particulate would. Other sources of gas-phase or vapor contamination are solvent residues resulting from reticle/pod cleaning operations during the photomask lifecycle, chemical agents. generated by out-gassing from the structural components of the carrier and chemical agents that enter into the carrier from the ambient atmosphere by breaching the hermetic sealing arrangement between the carrier shell and the carrier door. Multiple contamination species are thought to be the largest contributors to gas-phase contamination. These include NH3 (ammonia), SO2 (sulphur dioxide), H2O (moisture) and condensable organics C6-C10. Depending on the photolithography system, a photomask can be exposed to a laser light source of a wavelength that can range from 436 nm to 157 nm. Currently, 193 nm lasers are quite common. The energy of the laser can initiate chemical reactions that precipitate defect formation and propagation on the surface of the reticle. For instance, some of the chemical species are altered to form highly reactive species such as SO42− and NH4+. Some of these chemicals, such as acids for instance, are reactive with glass and can damage the reticle by etching it to create a haze on the patterned surface. The bases can create resist poisoning. The condensable organics can lead to SiC formation. In general, all of the contaminants can be considered to result in the same effect: crystal growth that degrades the functionality of the reticle. In this respect, the current thinking is that moisture or water is one of the key ingredients required for crystal growth. Essentially, water combines with some of the aforementioned contaminants to form the salts are generally clubbed together under the rubric of crystal growth. Prior art use of dessicants, for example, cannot ameliorate this problem because they cannot reduce the concentration of moisture to low enough levels to prevent salt (or crystal) formation. Likewise, purging a reticle carrier with clean dry air (CDA) or other dry gas may not reduce the moisture concentration to the levels required to avoid crystal growth. There is therefore a need for a contamination control mechanism at each stage of the reticle life cycle.
One of the approaches commonly employed in the art to ameliorate the effect of the chemical contaminants is periodic reticle/mask cleaning. The mean time between such cleans (MTBC) can approach, for example, approximately 8000 wafers in a 193 nm exposure tool. The threshold of the MTBC is set to prevent mean time between defects (MTBD) printed on the wafer using the reticle/mask. However, there is a limit to the number of such ‘cleans’ a reticle/mask can be subject to before resolution is degraded beyond functionality and the mask must be scrapped. In view of the above, one of skill in the art will recognize the need to ensure that the reticle environment within the carrier remains clean during storage, transportation, manipulation as well as during the standby condition when the carrier is empty of the reticle. While desirable, it is generally infeasible to construct a hermetically sealed environment that is absolutely impervious to incursion by AMCs or other contaminants. It is also infeasible to continuously purge the reticle carrier especially when the reticle and reticle carrier have to be transported or shipped.
What is needed is some type of structure or device for ensuring that the incursion, concentration and rate of accumulation of AMCs within the photomask carrier is controlled to levels that preclude or significantly reduce the formation of crystalline salts so that the useful life of the photomask can be significantly extended.